Working body panels is an art, no doubt about it. The best advise I received came from the fellow who taught me to work with lead and Bondo in 1973 (H.S. auto shop). He said as soon as you find there is a need to add filler to a panel, you become a sculptor. Once you realize that, you will also need to realize that you are no longer shaping the panel, you are sculpting the fill.
In other words, once you spread fill, forget the panel until you are happy with the shape of the fill, or when you *JUST* see the panel again as it's high spots poke through. At that point you have a decision to make. Either stop or add fill to the low spots and sculpt some more.
Most of the trick to sculpting anything is knowing when to stop removing material.